1. Field of the Invention
The present invention relates to a system for delivery of gas from a liquefied state, and to a method for delivery of gas at a high flow rate in a controlled manner.
2. Description of the Related Art
In the semiconductor manufacturing industry, high purity gases stored in cylinders are supplied to process tools for carrying out various semiconductor fabrication processes. Examples of such processes include diffusion, chemical vapor deposition (CVD), etching, sputtering and ion implantation. The gas cylinders are typically housed within gas cabinets. These gas cabinets also contain means for safely connecting the cylinders to respective process gas lines via a manifold. The process gas lines provide a conduit for the gases to be introduced to the various process tools.
Of the numerous gases utilized in the semiconductor manufacturing processes, many are stored in cylinders in a liquefied state. A partial list of chemicals stored in this manner, and the pressures under which they are typically stored, is provided below in Table 1:
TABLE 1 Vapor Pressure of Gas at 20.degree. C. Chemical Name Formula (psia) Ammonia NH.sub.3 129 Boron Trichloride BCl.sub.3 19 Carbon Dioxide CO.sub.2 845 Dichlorosilane SiH.sub.2 Cl.sub.2 24 Hydrogen Bromide HBr 335 Hydrogen Chloride HCl 628 Hydrogen Fluoride HF 16 Nitrous Oxide N.sub.2 O 760 Perfluoropropane C.sub.3 F.sub.8 115 Sulfur SF.sub.6 335 Hexafluoride Tungsten WF.sub.6 16 Hexafluoride
The primary purpose of the gas cabinet is to provide a safe vehicle for delivering gas from the cylinder to the process tool. The gas cabinet typically includes a gas panel with various flow control devices, valves, etc., in a configuration allowing cylinder changes and/or component replacement in a safe manner.
Variations in pressure of the relatively low volume cylinders stored in gas cabinets depend upon the rate of gas withdrawal from the cylinder (and the removal of the necessary heat of vaporization) as well as the transfer of ambient energy to the cylinder. Such effects are not typically present in bulk storage systems. In bulk storage systems, the thermal mass of the stored chemical is sufficiently large that liquid temperature variation occurs relatively slowly. Gas pressure in bulk systems is controlled by the temperature of the liquid. That is, the pressure inside the container is equal to the vapor pressure of the chemical at the temperature of the liquid.
In gas delivery systems based on cylinders, the need to control cylinder pressure by controlling cylinder temperature is recognized in the art. Gas cylinder heating/cooling jackets have been proposed for controlling cylinder pressure through the control of cylinder temperature. In such a case, a heating/cooling jacket can be placed in intimate contact with a cylinder. The jacket is maintained at a constant temperature by a circulating fluid, the temperature of which is controlled by an external heater/chiller unit. Such jackets are commercially available, for example, from Accurate Gas Control Systems, Inc.
Control of cylinder temperature coupled with thermal regulation of the entire gas piping system to prevent recondensation in the gas delivery system has been proposed for gases having low vapor pressures in U.S. Pat. No. 5,761,911 to Jurcik et al, hereby incorporated by reference in its entirety. The requirement for thermal regulation of the piping system is a result of the greater than ambient temperature of the cylinder caused by the heating/cooling jacket. If the gas line is not thermally controlled, recondensation of the gas flowing therethrough can occur when it passes from the heated zone into a lower temperature zone. Heating/cooling jackets coupled with thermal regulation is not favored due to the complications associated with system maintenance (e.g., during cylinder replacement) and the added expense.
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide a novel system for controlled delivery of gases from a liquefied state which allows for accurate control of the flow rate, while simultaneously providing safety features that allow shut down of the system at unacceptable temperatures for the gas employed.
It is a further object of the present invention to provide a method for delivery of gases at a variable flow rate and in a controlled manner.
It is another object of the present invention to provide a method for delivery of gases at a high flow rate over a period of time as required by the application carried out downstream.
It is yet another object of the present invention to provide a system for the delivery of a gas at consistently high purity and high flow rate, when needed and increasing the longevity of the cylinders from which the gas is withdrawn.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.